1. Field of the Invention
The present invention relates to hybrid working machines.
2. Description of the Related Art
In general, hybrid working machines such as hybrid shovels include a motor generator for assisting an engine. The motor generator is driven with electric power fed from an electric power accumulating unit including a rechargeable battery or an electric power accumulator. Further, electric power regenerated by a regeneration mechanism is stored in the electric power accumulating unit.
A large-capacity capacitor may be used as the electric power accumulator of the electric power accumulating unit. Such a capacitor has the characteristic of being degraded in an accelerating manner to have a reduced useful service life when repeatedly charged and discharged. Therefore, various techniques have been devised to extend the useful service life of the capacitor.
For example, in view of the fact that the degree of degradation is higher with a higher voltage of the capacitor, it has been proposed to discharge a capacitor at the end of the operation of a working machine so that the voltage of the capacitor becomes lower than or equal to a set voltage. The electric power discharged from the capacitor is accumulated in an electric power accumulating part such as an auxiliary battery.
Further, in order to discharge a capacitor without providing an electric power accumulating part such as an auxiliary battery, it has been proposed to reduce the voltage of the capacitor by consuming electric power by driving a water-cooling pump (an electric motor for a water-cooling pump) with electric power from the capacitor, for example. (See, for example, WO 2009/125833.)